Copper arsenate finds application in wood preservatives such as chromated copper arsenate. Chromated copper arsenate is generally made by dissolving the oxides of the constituent elements or by dissolving copper arsenate in chromic acid. Copper arsenate could be prepared by reacting solutions of a soluble arsenate with copper sulfate that are obtained from metallurgical processes that include leaching of arsenic-containing materials and copper-containing materials.
A number of processes for making copper arsenate have been disclosed such as in Canadian Patents 1 094 973 and 1 029 298, German Offen. DE 3 048 404 and U.S. Pat. No. 4 405 569.
The copper arsenate prepared from such processes has, generally, an impurity content that is too high to be acceptable under new specifications that require the total lead and iron content to be less than 0.1% by weight.
A method for the preparation of a low impurity-containing copper arsenate has been disclosed in co-pending U.S. Pat. Application No. 239 897 filed on Sept. 2, 1988 and assigned to the same assignee as the instant invention. According to this method, a solution of a soluble arsenate is added to copper sulfate solution, either or both solutions containing sludge-forming impurities, to form a copper and arsenic- containing solution without precipitating copper arsenate but precipitating the impurities at a controlled pH. After removal of precipitated impurities, the solution is neutralized to precipitate copper arsenate. Copper arsenate substantially free of impurities is recovered. In a preferred embodiment, the arsenate solution is formed by oxidative pressure leaching with sodium hydroxide of flue dusts, and copper sulfate is formed by acid oxidative pressure leaching of copper matte.
The method according to this co-pending application, wherein the copper and arsenic-containing solution is prepared from metallurgical starting materials, requires considerable amounts of sodium hydroxide and sulfuric acid to solubilize the arsenic as arsenate solution and the copper as copper sulfate solution. Another disadvantage is that the copper arsenate has an undesirably high water content when produced as a filter cake.